1. Field of the Invention
The present invention relates to fluorinated bis(phthalic anhydride) which is useful as intermediate raw materials of optical materials and the like, and a method for producing the same.
2. Description of the Related Art
Polyimide, which is highly fluorine-substituted, is useful as optical materials, wiring board materials, photosensitive materials, liquid crystal materials, or the like. This polyimide is produced by hydrolyzing a fluorine-substituted bis(phthalonitrile) compound to form a bis(phthalic acid) compound, dehydrating the bis(phthalic acid) compound to form bis(phthalic anhydride), then condensing the bis(phthalic anhydride) and a diamine compound to form polyamic acid, and further dehydrating the polyamic acid. However, various problems are caused when this production process are performed particularly in industrial mass synthesis.
For example, as the bis(phthalonitrile) compound which is a synthetic intermediate of polyimide, there is known 1,4-bis(3,4-dicyano-2,5,6-trifluorophenoxy)tetrafluorobenzene (hereinafter, referred to as “10 FEDN”). By this compound, excellent polyimide can be produced. This compound is generally obtained by following a synthetic route shown below and using 3,4,5,6-tetrafluorophthalonitrile (hereinafter, referred to as “TFPN”) and tetrafluorohydroquinone (hereinafter, referred to as “TFHQ”) as raw materials

In accordance with the above-mentioned synthetic route, it is thought that in theory, 2 equivalent amount of TFPN may be reacted with respect to TFHQ. However, under such a condition, a side reaction shown below occurs. That is, there was a problem that, as shown below, the compound formed by condensing TFPN and TFHQ in proportions of 1:1 is reacted not with the TFPN but with 10FEDN, which is an intended compound, and the yield and the purity of the intended compound (10FEDN) extremely deteriorated.

Technology described in Japanese Unexamined Patent Publications No. 6-16615 succeeded in attaining a fluorinated bis(phthalonitrile) compound having a high purity by using TFPN in an amount of 8 molar equivalent or more with respect to TFHQ. A method for producing a fluorinated bis(phthalonitrile) compound described in Japanese Unexamined Patent Publications No. 8-333322 is a method in which TFPN is also used in excess, and it facilitates purification by using a solvent which is hardly soluble in water.
In the purification processes employed in these conventional art, the intended compounds are purified with column chromatography after removing the raw material compound such as TFPN, which has not yet reacted and remains in excess in a reaction solution, by distillation. However, 10FEDN, which is an intended compound, is easily affected by heat, and therefore an operation temperature cannot be increased so much in a purification process. On the other hand, since TFPN, which is a raw material compound, has a melting point of 87° C. and is solid at room temperature, a reduced-pressure condition is required for removing by distillation. Therefore, when a fluorinated phthalonitrile derivative was produced by the conventional technology, it was necessary to perform the distillation under reduced-pressure in the purification process and this method was not suitable for mass synthesis at a plant level. Furthermore, there was a problem that since a distillation temperature could not be raised in the distillation under reduced-pressure in this purification process as described above, 10FEDN and others was solidified as the raw material compound was distilled off.
Thus, a method which avoids a distillation process has been desired in the purification process in the production of the fluorinated bis(phthalonitrile) compound. However, column chromatography and the like, which is a purification technique other than distillation, have the problems of an equipment cost and the like. Accordingly, as a purification process for the mass synthesis of the fluorinated bis(phthalonitrile) compound, a purification by recrystallization is ideal.
However, according to the rule of common sense in the synthetic organic chemical field, a compound having less impurities (particularly impurities having a structure similar to these compound) is apt to crystallization. Conversely, when a raw material compound having a structure in common with the intended compound is present as an impurity in a large amount in a reaction system, purification by recrystallization of the intended compound is difficult. Therefore, in the production of the fluorinated bis(phthalonitrile) compound, where the raw material compound is remaining excessively in a reaction system after the completion of a reaction, it was considered to be difficult that a recrystallization process is applied to the purification process.
Also, a process, in which a bis(phthalic anhydride) is produced by dehydrating a bis(phthalic acid) compound, has also a problem. It is thought that as such the dehydration reaction, a known method is applied. As an examples of such the known method, there is described a method, in which only acetic anhydride is added to a mixture of 2,3,3′,4′-biphenyl-tetracarboxylic acid and 3,4,3′,4′-biphenyltetracarboxylic acid and this mixture is heated to obtain biphenyltetracarboxylic anhydride, in a example in Japanese Unexamined Patent Publications No. 51-23498. Similarly, in the technology described in Japanese Unexamined Patent Publications No. 62-116572, 3,4,3′,4′-biphenyltetracarboxylic anhydride is produced using acetic anhydride as a solvent, and the concentration of 3,4,3′,4′-biphenyltetracarboxylic acid, which is a raw material compound, is specified for the purpose of improving handling through an increase in a crystal size and reducing coloration contents.
However, if the methods of these patent references are applied to the production of the fluorinated bis(phthalic anhydride) as it is, not only yield may deteriorates due to the occurrence of a side reaction but also the intended compound may colored. It is conceivable as one of causes that since a fluorinated bis(phthalic acid) compound, which is a raw material compound, exhibits extremely high solubility in acetic anhydride or acetic acid, a reaction proceeds excessively and a dehydration reaction occurs between the raw material compounds instead of the dehydration between adjacent carboxyl groups in a molecule. On the other hand, when the concentration of raw material compounds in acetic anhydride are decreased in order to suppress such the side reaction as with the technology described in Japanese Unexamined Patent Publications No. 62-116572, purification of the intended compound is difficult and the loss of the intended compound becomes greater. Furthermore, this technology requires contacting with an organic solvent prior to the dehydration reaction in order to enhance an effect.
A method for producing fluorinated phthalic anhydride by dehydrating phthalic acid, which is highly fluorine-substituted, is described, for example, in Japanese Unexamined Patent Publications No. 2-306945. That is, there is disclosed in this reference a method of obtaining tetrafluorophthalic anhydride by heating tetrafluorophthalic acid in the presence of an organic solvent such as xylene or toluene. However, in case where this method is applied to the production of the fluorinated bis(phthalic anhydride), since the fluorinated bis(phthalic acid) compound, which is a raw material compound, has relatively low solubility in xylene and the like, a reaction must be a slurry reaction and therefore it may take too much time to react or the intended compound having a high purity may not attained by using only xylene and the like. Therefore, this method cannot be adopted as a method of mass synthesis of the fluorinated bis(phthalic anhydride).
On the other hand, in Japanese Patent Publications No. 3130653, there is disclosed a method of synthesizing the fluorinated bis(phthalic anhydride). In this method, dehydration is performed by heating the fluorinated bis(phthalic acid) compound in a solvent such as thionyl chloride, and this method accomplishes certain results. However, since this reaction is carried out in a slurry condition, optimal reaction conditions varies due to a small difference in conditions, for example, a reaction time varies significantly with reaction scales or material lots. Further, since a extremely reactive dehydrating agent such as thionyl chloride is used, the intended compound is readily colored and it is difficult to attain products of high quality. Furthermore, there is also a problem that the fluorinated bis(phthalic anhydride) obtained by this method has a small specific surface area and is not insufficient in solubility.
As described above, there are known methods of synthesizing the fluorinated bis(phthalonitrile) compound and the fluorinated bis(phthalic anhydride), which is a synthetic intermediate of highly fluorine-substituted polyimide, and methods considered to be applicable to its synthesis.
However, in the conventional methods, efficiency was low and it was difficult to expand the scale of implementation to a plant level of mass synthesis. Additionally, since the fluorinated bis(phthalic anhydride) attained by the conventional methods has a small specific surface area and low solubility, an efficiency at the next step may deteriorates. Further, it becomes important to reduce further a coloration of a synthetic intermediate, since particularly when polyimide, final products, is used as optical materials, the coloration becomes a problem.